Further tests of permeability with low hydraulic gradients

Many of the water‐bearing formations in the United States have hydraulic gradients of much less than 20 feet to the mile, and some may have gradients of less than one foot to the mile, whereas most laboratory‐tests of permeability are made with much higher gradients. An investigation was therefore undertaken by the writer, under the direction of 0. E. Meinzer, in the Hydrologic Laboratory of the United States Geological Survey, to determine whether Darcy's law is valid for very low hydraulic gradients—that is, whether under such low gradients the rate of flow remains proportional to the gradient. In a previous paper by Meinzer and the writer (Amer. Geophysical Union,15th annual meeting, pp.405–409, 1934), it was shown that for the sand tested the rate of flow varies directly with the hydraulic gradient down to a gradient of about one foot to the mile. More recently six additional tests have been made on this sand with the same apparatus, and these tests give evidence of the validity of Darcy's law for still lower gradients.